Overview of Metabolic Reactions

• Metabolic processes are constantly taking place in the body.

• Metabolism is the sum of all of the chemical reactions that are involved in catabolism and anabolism.

• Catabolic reactions break down large organic molecules into smaller molecules, releasing the energy contained in the chemical bonds.

• In contrast to catabolic reactions, anabolic reactions involve the joining of smaller molecules into larger ones. Anabolic reactions combine monosaccharides to form polysaccharides, fatty acids to form triglycerides, amino acids to form proteins, and nucleotides to form nucleic acids.

• Catabolic hormones stimulate the breakdown of molecules and the production of energy.

• Anabolic hormones are required for the synthesis of molecules and include growth hormone, insulin-like growth factor, insulin, testosterone, and estrogen.

Oxidation-Reduction Reactions

• The loss of an electron, or oxidation, releases a small amount of energy; both the electron and the energy are then passed to another molecule in the process of reduction, or the gaining of an electron.

• An oxidation-reduction reaction (also called a redox reaction)—when an electron is passed between molecules, the donor is oxidized and the recipient is reduced.

• The two most common coenzymes of oxidation-reduction reactions are nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD).

Carbohydrate Metabolism

• The complex sugars are also called polysaccharides and are made of multiple monosaccharide molecules.

• Carbohydrate digestion begins in the mouth with the action of salivary amylase on starches and ends with monosaccharides being absorbed across the epithelium of the small intestine.

• Once the absorbed monosaccharides are transported to the tissues, the process of cellular respiration begins.

Glycolysis

• Cells in the body take up the circulating glucose in response to insulin and, through a series of reactions called glycolysis, transfer some of the energy in glucose to ADP to form ATP

• The last step in glycolysis produces the product pyruvate.

• In the presence of oxygen, pyruvate continues on to the Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle (TCA), where additional energy is extracted and passed on.

• When glucose enters a cell, the enzyme hexokinase (or glucokinase, in the liver) rapidly adds a phosphate to convert it into glucose-6-phosphate.

• Hexokinase is found in nearly every tissue in the body.

• Glucokinase, on the other hand, is expressed in tissues that are active when blood glucose levels are high, such as the liver.

Krebs Cycle/Citric Acid Cycle/Tricarboxylic Acid Cycle

• The pyruvate molecules generated during glycolysis are transported across the mitochondrial membrane into the inner mitochondrial matrix, where they are metabolized by enzymes in a pathway called the Krebs cycle.

• The three-carbon pyruvate molecule generated during glycolysis moves from the cytoplasm into the mitochondrial matrix, where it is converted by the enzyme pyruvate dehydrogenase into a two-carbon acetyl coenzyme A (acetyl CoA) molecule.

Oxidative Phosphorylation and the Electron Transport Chain

• The electron transport chain (ETC) uses the NADH and FADH2 produced by the Krebs cycle to generate ATP.

• The ETC couples the transfer of electrons between a donor (like NADH) and an electron acceptor (like O2) with the transfer of protons (H+ ions) across the inner mitochondrial membrane, enabling the process of oxidative phosphorylation.

• Also embedded in the inner mitochondrial membrane is an amazing protein pore complex called ATP synthase.

• Gluconeogenesis: Gluconeogenesis is the synthesis of new glucose molecules from pyruvate, lactate, glycerol, or the amino acids alanine or glutamine.

Lipid Metabolism

• Lipid metabolism begins in the intestine where ingested triglycerides are broken down into smaller chain fatty acids and subsequently into monoglyceride molecules by pancreatic lipases, enzymes that break down fats after they are emulsified by bile salts.

• Within the intestinal cells, these triglycerides are packaged along with cholesterol molecules in phospholipid vesicles called chylomicrons.

Lipolysis

• To obtain energy from fat, triglycerides must first be broken down by hydrolysis into their two principal components, fatty acids and glycerol. This process, called lipolysis, takes place in the cytoplasm.

• The breakdown of fatty acids, called fatty acid oxidation or beta (β)-oxidation, begins in the cytoplasm, where fatty acids are converted into fatty acyl CoA molecules.

Ketogenesis

• If excessive acetyl CoA is created from the oxidation of fatty acids and the Krebs cycle is overloaded and cannot handle it, the acetyl CoA is diverted to create ketone bodies.

• In this ketone synthesis reaction, excess acetyl CoA is converted into hydroxymethylglutaryl CoA (HMG CoA).

• Ketone Body Oxidation: Ketones oxidize to produce energy for the brain. beta (β)-hydroxybutyrate is oxidized to acetoacetate and NADH is released.

Lipogenesis

• When glucose levels are plentiful, the excess acetyl CoA generated by glycolysis can be converted into fatty acids, triglycerides, cholesterol, steroids, and bile salts.

  • This process, called lipogenesis, creates lipids (fat) from the acetyl CoA and takes place in the cytoplasm of adipocytes (fat cells) and hepatocytes (liver cells).

Protein Metabolism

• When protein-rich foods enter the stomach, they are greeted by a mixture of the enzyme pepsin and hydrochloric acid (HCl; 0.5 percent).

• When the food-gastric juice mixture (chyme) enters the small intestine, the pancreas releases sodium bicarbonate to neutralize the HCl.

• The small intestine also releases digestive hormones, including secretin and CCK, which stimulate digestive processes to break down the proteins further.

• The pancreas releases most of the digestive enzymes, including the proteases trypsin, chymotrypsin, and elastase, which aid protein digestion.

• Trypsin and chymotrypsin break down large proteins into smaller peptides, a process called proteolysis.

• Urea Cycle: The urea cycle is a set of biochemical reactions that produces urea from ammonium ions in order to prevent a toxic level of ammonium in the body.

Metabolic States of the Body

• The absorptive state, or the fed state, occurs after a meal when your body is digesting the food and absorbing the nutrients (catabolism exceeds anabolism).

• The postabsorptive state, or the fasting state, occurs when the food has been digested, absorbed, and stored.During this state, the body must rely initially on stored glycogen.

Energy and Heat Balance

• The body tightly regulates the body temperature through a process called thermoregulation, in which the body can maintain its temperature within certain boundaries, even when the surrounding temperature is very different.

• An environment is said to be thermoneutral when the body does not expend or release energy to maintain its core temperature.

Mechanisms of Heat Exchange

• Conduction is the transfer of heat by two objects that are in direct contact with one another.

• Convection is the transfer of heat to the air surrounding the skin.

• Radiation is the transfer of heat via infrared waves.

• Evaporation is the transfer of heat by the evaporation of water.

Metabolic Rate

• The metabolic rate is the amount of energy consumed minus the amount of energy expended by the body.

• The basal metabolic rate (BMR) describes the amount of daily energy expended by humans at rest, in a neutrally temperate environment, while in the postabsorptive state.

Nutrition and Diet

• The nutritional Calorie (C) is the amount of heat it takes to raise 1 kg (1000 g) of water by 1 °C.

• Vitamins are organic compounds found in foods and are a necessary part of the biochemical reactions in the body.

• Minerals in food are inorganic compounds that work with other nutrients to ensure the body functions properly.